1. Field of the Invention
This invention relates to panels for forming a floor covering and to a method for manufacturing such panels.
More particularly, the invention relates to panels comprising at least a layer, more particularly a substrate layer, of thermoplastic material. Further, such panels may also comprise, above said layer, at least a top layer with a printed decor and a translucent or transparent wear layer.
In particular, the present invention relates to panels of the type which, on two or more opposite edges, comprises coupling means or coupling parts with which two of such floor panels can be coupled at said edges in such a manner that they are mutually locked in a horizontal direction perpendicular to the respective edge and in the plane of the floor panels, as well as in a vertical direction perpendicular to the plane of the floor panels. Such floor panels can be applied for composing a so-called floating floor covering, wherein the floor panels are connected to each other at their edges, however, lie freely on the underlying surface.
2. Related Art
Such panels are known, for example, from EP 1 938 963, EP 2 339 092 A1 and WO 2010/023042. From these documents, more particularly vinyl-based floor panels are known for forming a floating floor covering. Such vinyl-based floor panels mostly have a thickness of 3 to 5 millimeters and have a high material density. The respective core of the panels is composed of one or more prefabricated layers of thermoplastic material, for example, soft PVC. Above the print, a transparent thermoplastic layer and possibly a superficial layer of lacquer are provided. The various layers are adhered to each other by means of a thermal lamination process. According to EP 2 339 092 A1, a reinforcing layer on the basis of glass fiber can be applied.
The panels of the state of the art show the disadvantage that they are strongly subjected to shrinkage and expansion, or even bending, under the influence of temperature. Shrinkage and expansion may lead to that the floor covering or the individual panels are pushed upward, and, for example, to a mutual release of the coupling parts, or to the phenomenon that the individual panel-shaped elements will drift apart, whereby gaps may occur. The features of the panels of the state of the art are such that problems may occur already with warming due to incident sunlight, for example, at a window in a room.
From WO 2012/004701, it is known to apply a glass fiber layer or embedded profiles for increasing the dimensional stability of such panels. In particular, here the bending stiffness of the panels has been tackled.
U.S. Pat. No. 6,306,318 describes a method for recycling discarded carpets, wherein, starting from a molten mixture of a recyclate on the basis of discarded carpets and fresh soft PVC, flexible floor tiles having a minimum thickness of 5 millimeters are produced by means of an injection molding process. As an alternative for the recyclate of discarded carpets, also recyclate of glass fiber-reinforced synthetic material (English: fiberglass) can be applied. The obtained tile is relatively hard and the flexibility leaves much to be desired. This may lead to problems with the impact sound during use and an incapability of adapting to unevennesses of the underlying surface. The injection-molded tile comprises coupling means along the edges, however, these do not provide for a locking in vertical direction. Moreover, the precision of injection molding, in particular in the case of soft thermoplastic materials, is inferior. This method is time-consuming, and moreover there is little possibility of variation in the appearance of the wear side of the tiles. Such tiles possibly still show a strong dimensional change with changing temperature.
GB 1 248 060 describes, as an alternative of a method similar to that of U.S. Pat. No. 6,306,318, the possibility of manufacturing reinforced thermoplastic materials by means of a method wherein continuous fiber layers are stacked and pressed in a mold alternating with thermoplastic granulate layers. According to the examples, hard PVC can be applied as thermoplastic material. Above all, this document aims at an increased bending stiffness of the obtained thermoplastic material. This method, too, is time-consuming and offers little possibility of variance in the appearance of the obtained materials.